1. Field of the Invention
The present invention relates to a linear transmitter for providing amplitude and phase modulation to an input signal. More particularly, the present invention pertains to a device and method for generating a driver signal for driving a non-linear amplifier and for producing linear amplification.
2. Description of the Related Art
Many modulation schemes for transmitting modulated phase and modulated amplitude components of an input signal in communications systems are known. In some schemes, linear transmitters employing linear amplifiers are used because such amplifiers preserve the phase and amplitude components of the input signal and, consequently, the data contained therein. The drawback of using linear amplifiers, however, is the low power efficiency associated with such amplifiers. Other prior art schemes attempt to simulate linear transmitter performance using non-linear amplifiers so as to benefit from the higher power efficiency that non-linear amplifiers exhibit. The problem with these alternate schemes is that although non-linear amplifiers exhibit increased power efficiency over linear amplifiers, they lose or distort amplitude information carried by the amplitude component of the input signal.
In an attempt to compensate for or prevent amplitude loss in the use of non-linear amplifiers, some prior art systems isolate the phase and amplitude components from each other and the phase component is then used to drive a voltage controlled oscillator (VCO) to produce a phase modulated VCO signal. Techniques are then used to incorporate or otherwise add the amplified amplitude component to the amplified phase component. The particular amplitude incorporation technique will vary depending on the type of non-linear amplifier used, e.g. a Class C amplifier or a Class D amplifier will employ a switching mechanism for switching amplifiers in and out of an amplifier array, or other amplitude restoration techniques to restore lost or distorted amplitude information. Although such techniques exist for Class C and Class D amplifiers, no simple techniques presently exist which allow for the use of highly efficient Class S amplifiers in a linear transmitter so as to permit the incorporation of amplitude information into an existing phase modulated signal.